The present invention is generally related to satellite communications systems and, more particularly, to a constellation of non-geostationary satellites that can be deployed and utilized in a manner that materially increases global communications satellite capacity, does not interfere with the existing geostationary satellite ring, and provides simplified satellite tracking.
Geostationary (xe2x80x9cgeoxe2x80x9d) satellites for telecommunications applications were first proposed many years ago by the author Arthur C. Clark. Today, there are numerous communications systems employing geo satellites for such diverse applications as telephone and data trunking, television distribution, direct-to-home broadcasting, and mobile communications. Geo satellites operate on the physical principle that a satellite, in circular orbit at the proper altitude above the equator, will orbit the earth at the same angular velocity as the earth""s rotation. These satellites therefore, appear to be fixed relative to a point on the earth. This characteristic of geo satellites facilitates their use for communications applications by allowing communications terminals on the earth to simply point their antennas at essentially one position in the sky.
There are however, a number of distinct drawbacks associated with geostationary satellite systems. One major drawback is the high cost of raising a satellite into geo orbit. Geostationary orbits have a radius from the earth center of approximately 36,000 kilometers. Typically, a geo satellite is launched first into an elliptical transfer orbit having an apogee at geostationary altitude, and then its orbit is circularized by using a kick motor to impart the necessary additional momentum to the satellite at apogee. The apogee kick motor, before it is fired, typically weighs as much as the satellite itself, meaning that the launch vehicle must initially launch a payload twice as heavy as the satellite in final orbit. Accordingly, the cost of putting a satellite into the high circular orbit required for geostationary operation is significantly greater than for non-geostationary satellites. The cost associated with deployment of satellites must generally be amortized over the lifetime of the satellite, making use of geo satellites more expensive.
The high altitude of the geostationary orbit also adds to the size and weight of geo satellites. Path loss, the attenuation suffered by radio signals traveling in free space, is proportional to the square of the distance between the source and the receiver. This means that the antenna size and transmitted power of a geo satellite must be greater than those of a satellite in lower orbit in order to achieve the same communications link performance. This is particularly true in mobile and other direct-to-user applications where the size and power of the user terminal are constrained by practical considerations and the burden of providing acceptable link performance falls largely on the satellite. The generally larger size and weight of geo satellites adds further to the cost of launch as compared to satellites intended to operate in lower orbits.
Another problem associated with the altitude at which geo satellites orbit is the delay in the round trip transmission to and from the satellite. For a pair of diverse communications terminals located within the coverage area of a gee satellite, the path length from terminal-to-satellite-to terminal is at least 70,000 kilometers. For the average satellite xe2x80x9chopxe2x80x9d the associated transmission delay is approximately one-quarter of a second. For voice communications by satellite, the delay is noticeable to some users, and may require the use special circuitry for echo control. For data communications, the delay complicates the use of protocols that are predicated on the characteristics of terrestrial circuits.
Other problems arise from the geometry of coverage of geo satellite systems. A geostationary satellite system intended to provide xe2x80x9cglobalxe2x80x9d services would include three geo satellites spaced equal along the equatorial arc at 120-degree intervals. The coverage area of each of these satellites describes a circle on the surface of the earth with its center on the equator. At the equator, the coverage areas of two adjacent geo satellites overlap approximately 40 degrees in longitude. However, the overlap decreases as latitude increases, and there are points on the earth, north and south of the coverage areas, from which none of the geo satellites is visible. For example, many points in Alaska, Canada and Scandinavia cannot even see the geo satellites, these satellites being below their visible horizon.
For a geo system, in which the satellites are in orbit above the equator, earth stations in the equatorial regions generally xe2x80x9cseexe2x80x9d the satellites at high elevation angles above the horizon. However, as the latitude of an earth station location increases, the elevation angle to geo satellites from the earth station decreases. For example, elevation angles from ground stations in the United States to geostationary satellites range from 20 to 50 degrees. Low elevation angles can degrade the satellite communications link in several ways. The significant increase in path length through the atmosphere at low elevation angles exacerbates such effects as rain fading, atmospheric absorption and scintillation. For mobile communications systems in particular, low elevation angles increase link degradation due to blockage and multi-path effects.
Another, and perhaps more significant, problem resulting from the specific geometry of the geo orbit, is the limited availability of orbital positions (or xe2x80x9cslotsxe2x80x9d) along the geostationary orbital arc. The ring of geostationary satellites that has grown up over time generally occupies multiple slots spaced two degrees apart and identified by their longitudinal positions. This arrangement has been adopted internationally to allow for satellite communications with a minimum of interference between adjacent satellites operating in the same frequency bands. The two-degree spacing is achieved by using high gain, directional antennas at the ground stations accessing the satellites. The geo ring around the equator thus provides a total of 180 slots (360 degrees/two degrees per slot). Most of the geo slots are now occupied, making it difficult to find positions for more geo satellites. Frequency, polarization and beam diversity have been used to multiply capacity, but capacity in the geostationary arc remains limited. Moreover, not all geo orbital positions are equally useful or attractive for various applications.
Various non-geostationary satellite systems have been implemented in the past to overcome some of the drawbacks of geo satellites. An early example is the Russian Molniya system, which employed satellites in elliptical 12-hour orbits to provide coverage to the northern latitudes in the Soviet Union. The Iridium and Globalstar systems use satellites in low circular orbits to significantly reduce transmission delay and allow acceptable link performance with very small user terminals. However, non-geostationary systems operate in inclined orbits, and thus pose a potential for interference with geo satellites operating at the same frequencies as they cross the geostationary ring.
In January 1999, an application was filed before the Federal Communications Commission (FCC) by Virtual Geosatellite LLC for the construction of a global broadband satellite communications system based on the teachings of U.S. Pat. Nos. 5,845,206 and 5,957,409, issued on Dec. 21, 1998 and Sep. 28, 1999, respectively, to the inventor of the present invention and two other individuals. The system proposed in the FCC application employs three arrays of satellites in elliptical orbits, two arrays covering the northern hemisphere and one covering the southern hemisphere, each array having five 8-hour satellites emulating many of the characteristics of geo satellites. The satellites appear to xe2x80x9changxe2x80x9d in the sky because their angular velocity at or near apogee approximates the rotation rate of the earth. Nine so-called xe2x80x9cactive arcsxe2x80x9d are created with centers located at the apogee points of the satellite orbits. The satellites in each of the three arrays move in a repeating ground track from one active arc to the next, so that there is always one active satellite available in each active arc. While in their active arcs, the satellites move very slowly, averaging only about eight degrees per hour, with respect to terrestrial antennas. Between arcs, the satellites are deactivated. The active arcs occupy a different portion of the sky than any of the geo satellites located near the equator. As a result, the virtual geo satellites are visible from most parts of the northern and southern hemispheres, but do not interfere with satellites in the geo arc.
Although the prior art virtual gee satellite constellation described above addresses many of the shortcomings of geostationary satellites, it requires that ground terminals track the satellites as they slowly traverse the active arcs. Moreover, as one satellite leaves the end of an active arc and is deactivated, the ground station antenna must quickly re-point, or slew, 40-50 degrees to pick up the satellite that has just arrived at the beginning of the active arc to take the place of the first satellite. For large antennas, such rapid slewing may prove impractical, and actually require the use of two antennas at each site. Phased array antennas can provide rapid re-pointing, but the commercial availability of affordable designs, especially for the consumer market, is unclear. Some form of data buffering to cover the outage period is another possible alternative, although also likely to be complex and expensive.
Therefore, it is an objective of the present invention to provide a system of non-geostationary satellites that significantly simplifies the tracking requirements and reduces the cost for satellite ground stations.
It is another objective of the present invention to provide a system of satellites that materially increases global communications satellite capacity without interfering with the existing geostationary satellite ring.
It is a further objective of the present invention to provide a global system of communications satellites with higher average elevation angles and lower transmission delay than existing geostationary satellites.
It is yet a further objective of the present invention to provide a total global communications system of satellites and ground facilities with lower construction and implementation costs than existing geostationary systems.
The above-stated objectives, as well as other objectives, features and advantages, of the present invention will become readily apparent from the following detailed description, which is to be read in conjunction with the appended drawings.
The present invention is directed to a constellation of non-geostationary satellites that may be deployed and utilized in a manner that materially increases global communications capacity, does not interfere with satellites in the existing geostationary ring, and provides simplified satellite tracking. A system embodiment includes first and second pluralities of satellites in inclined elliptical orbits, each plurality of satellites forming a repeating ground track that brings the satellites over the same points on the earth everyday. In the preferred embodiment the satellites have a mean motion of 3, meaning they orbit the earth three times per day, but other integer values of mean motion, such as 2 and 4 are applicable.
Each orbiting satellite has communications equipment on board for communicating with ground stations. The communications equipment on each satellite in the constellation is enabled, or activated (e.g., powered) only during a portion of the orbit when the satellite is near apogee, the point in the orbit where the satellite altitude is greatest and the satellite is moving most slowly from the viewpoint of the earth stations. Preferably, the portion of the orbit during which the satellite is enabled is symmetrically disposed about the apogee of the orbit. In the preferred embodiment, with mean motion 3, each of the satellites is enabled near its apogee for a duration of 4 hours, which is 50 percent of its total orbit period.
Each of the satellite ground tracks has a number of active arcs corresponding to the portion of the satellite orbits during which the communications equipment on the satellites is enabled to communicate. The orbits of the first plurality of satellites are configured such that each of the active arcs of the first ground track begins and ends at points that fall on the same meridian of longitude. This is accomplished by selecting an argument of perigee that xe2x80x9cleansxe2x80x9d the satellite orbits toward the equator, placing the satellite apogee at about 40 degrees latitude. The xe2x80x9cargument of perigeexe2x80x9d is an orbital parameter that indicates the angular position in the plane of the orbit where perigee occurs. Arguments of perigee between zero degrees and 180 degrees locate the position of perigee in the Northern Hemisphere, and hence concentrate satellite coverage in the Southern Hemisphere. Conversely arguments of perigee between 180 degrees and 360 degrees locate the perigee in the Southern Hemisphere and hence concentrate coverage on the Northern Hemisphere.
At the same time, the orbits of the second plurality of satellites have an argument of perigee that is the supplementary angle of the argument of perigee of the first plurality of satellites, causing the satellite orbits of the second plurality of satellites to lean by an equal amount in the opposite direction. The orbits of the second plurality of satellites are further configured such that each active arc of the second ground track begins at a point coincident with the ending point of one of the active arcs of the first ground track, and ends at a point coincident with the beginning point of the same one of the first active arcs. The result, as viewed from a ground station, is a closed path formed by an active arc of the first ground track and a corresponding active arc of the second ground track. For the preferred embodiment with an orbital mean motion of three, the closed path is repeated three times around the earth, at equal, 120-degree intervals.
In addition to the constellation of satellites, the system embodiment of the present invention typically includes a plurality of ground stations, each having communications equipment configured to communicate with the communications equipment on the first and second plurality of satellites, and located at positions on the earth from which they can track satellites in one of the first active arcs and satellites in the one second active arc that has coincident beginning and ending points.
In another aspect of the invention, the orbits of the first and second pluralities of satellites are configured such that at all times there is at least one satellite in either each of the active arcs of the first ground track, or each of the active arcs of the second ground track. Preferably, there are an equal number of satellites in the two ground tracks, and the orbits of the satellites are further configured such that when one satellite is at the end of an active arc in one ground track and in the process of being deactivated, another satellite is at or near the beginning of the corresponding active arc in the other ground track and being reactivated. At the changeover point the two satellites must be near enough to allow a ground station to follow what appears to be a single active satellite in a closed path in the sky overhead without having to break lock and slew to a new position when satellite changeovers occur. However, the orbital parameters of the satellites in the first and second ground tracks are preferably selected such that at the points where the ground tracks cross, the satellites are far enough apart in space that they do not actually collide.
Preferably, the satellites in each ground track are equally spaced in mean anomaly to achieve the greatest number of satellites enabled at the same time. xe2x80x9cMean anomalyxe2x80x9d represents the fraction of an orbit period that has elapsed since the satellite passed through perigee, as expressed in degrees. For example, the mean anomaly of a satellite two hours into an 8-hour orbit is 90 degrees (one quarter of the period).
Continuous communication at the preferred 50 percent duty cycle requires a minimum of three evenly spaced satellites in each ground track. Adding more basic groups of six satellites to the two ground tracks creates additional orbital capacity. In the preferred embodiment, the orbital parameters allow up 12 satellites to be placed in each active arc of the ground track while maintaining a minimum angular spacing between satellites of at least 2 degrees.
In another aspect of the invention, each of the satellites in the constellation has an orbital height lower than the height necessary for geostationary orbits. This aspect of the invention has the benefit of reducing satellite size and weight for a given communications capacity, reducing launch requirements, and reducing satellite transmission delay. Also launching into elliptical orbits requires less energy than circular orbits, further reducing launch vehicle costs.
To minimize perturbation effects caused by the earth""s shape, the present invention also preferably uses the critical orbital inclination of 63.4 degrees. This is the inclination of the orbital plane that results in a stable elliptical orbit whose apogee always stays at the same latitude in the same hemisphere.
In another aspect of the present invention, the orbits of satellites are configured such that the portion of the satellites"" orbits during which communications equipment is enabled, is separated from the earth""s equatorial plane by at least a predetermined amount. This feature avoids potential interference with existing satellites in the geostationary ring and allows the communications frequencies allocated to geostationary satellites to be reused for the non-geostationary constellation of the present invention.
In a further aspect of the present invention, each satellite has a power system configured to generate an amount of power less than that required when the communications equipment on the satellite is enabled, and more than that required when the communications equipment is not enabled. The power system can store the excess power generated when the communications equipment is not enabled, and use the stored power to supplement the generated power to meet the requirements of the communications equipment when it is enabled. For the preferred embodiment with a duty cycle of 50 percent, satellite weight saving resulting from this power conservation scheme can be significant.
In yet a further aspect of the invention, satellites are added to the constellation to form additional pairs of ground tracks having the same shapes as the first and second ground tracks, but displaced in longitude by a predetermined amount. The amount of longitudinal displacement is such that at all times each of the satellites in the active arcs of the additional ground track pairs is separated by at least a predetermined angle from any of the active satellites in other ground track pairs. The preferred embodiment, with satellites in orbits having a mean motion of three and operating at a 50 percent duty factor, can accommodate four pairs of ground tracks having 24 active arcs (i.e., 12 closed paths) in each hemisphere, or a total of 48 active arcs worldwide. If each arc is filled with a maximum of 12 active satellites, the total number of equivalent non-geostationary satellite slots that the present invention can support is 576, or more than three times as many as the existing geo stationary ring, assuming minimum two-degree satellite spacing.